This invention relates to ring laser gyroscopes in general and more particularly to circuitry which operates in conjunction with piezoelectric actuators for providing an improved path length controller to use with ring laser gyroscopes.
Because of the complex requirements of todays military and space flight equipment, greater and greater demands are being placed on such equipment as guidance systems. Since gyroscopes represent an essential part of most such systems, these stringent demands are also required of the gyroscope itself. Therefore, over the years many types of gyroscopes have been developed to meet these increasing demands. One sophisticated modern type gyroscope is referred to as "a ring laser gyroscope". As is inherent in its name, the ring laser gyroscope uses a laser beam which travels in a closed path. Regardless of whether the closed path is triangular, square, octagon, etc., the closed path is commonly referred to as a ring. Such a ring laser gyroscope is used to detect rotation about the axis of the path around which the laser beam travels. Typical ring laser gyroscopes are disclosed in U.S. Pat. Nos. 3,373,650 and 3,467,472. Again, because of the harsh environments experienced by modern guidance systems, the ring laser gyroscope must operate over a wide range of temperatures, and as a result, the material of which a gyroscope is made suffers thermal expansion and contractions as the temperature varies. As an example, the temperature variation may range from -55.degree. C. to +70.degree. C. Since the laser beam of the ring laser gyroscope is normally directed along its path by means of mirrors, such thermal expansion and contraction of either of the supporting structure for the mirrors or the mirrors themselves will cause a change in the path length. Although the path followed by the laser beam or a ring laser gyroscope is commonly referred as "a ring", it will be appreciated as was mentioned before, that the path is typically of a triangular shape, since a triangular shaped path constitutes the smallest number of direction changes which can result in a closed path. In any event, regardless of whether the path of the laser beam is triangular, square or some other shape, any change in the path length due to thermal expansion or contraction, if not corrected, may well result in drift by the gyroscope. That is, the gyroscope output will indicate a rotation has occurred when in fact none actually occured. In a typical triangular ring laser gyroscope this problem is often solved by mounting one of the reflecting surfaces, typically a mirror, such that its location can be slightly varied as necessary, to maintain the path length constant even though temperature changes makes the material expand or contract. This may be accomplished by constructing one mirror or reflecting surface with a flexible annulus which is attached to a piezoelectric actuator. The piezoelectric actuator is then used to maintain the path length of the laser constant by deforming the mirror and thereby changing the position of the reflecting surface. The piezoelectric actuator operates in response to detected changes in the ring laser path length, and thus a closed loop serve system is obtained.
The ring laser gyroscope shown and described in U.S. Pat. Nos. 3,373,650 and 3,467,472 include a triangular block which forms a triangular shaped ring laser cavity defined by mirrors at the three corners. It will be appreciated that the triangular-shaped block is preferred since it requires a minimum number of mirrors. The laser cavity itself is filled by a gas which comprises, for example, helium and neon. The laser usually operates at one of two wave lengths; specifically either at 1.15 micrometers in the infrared spectral band or at 0.63 micrometers in the visible wave length region. Through proper choice of the ratios of the two neon isotopes Ne.sup.20 and Ne.sup.22 in the gas mixture, two monochromatic laser beams are created. The two laser beams respectively travel in clockwise and counterclockwise directions around the triangular cavity in the same closed optical path.
With no angular motion about the input axis of the ring laser gyroscope, the lengths of the two laser beams are equal, and the two optical frequencies are the same. Angular movement in either direction about its input axes, however, causes an apparent increase in the cavity length for the beam travelling in the direction of such angular movement and a corresponding decrease for the beam travelling in the opposite direction. Because the closed optical path is a resonant cavity providing substained oscillation, the wave length of each beam must also be increased or decreased accordingly. Angular movement of the ring laser gyroscope in either direction about its input axes, therefore, causes a frequency differential to occur between the two beam frequencies which is proportional to the angular rate.
In accordance with the prior art practice, the two beams are extracted from the laser at its output mirror and they are heterodyned in a beam combiner to produce an interference pattern. The interference pattern is detected by a photodetector which senses the beam frequency of the heterodyned optical frequencies of the two beams, and this beam frequency is the measure of the angular rate.
However, regardless of whether a ring laser gyroscope of a type just discussed or even another method is used. as was discussed heretofore, the ring laser gyroscope must be capable of operating over a wide range of temperatures.
Typically, the piezoelectric actuators used to maintain the path length of the laser are designed to control the path length to an integral number of laser wave lengths. It is usually necessary that an actuator at least have the ability to change the flexible mirror five free spectral ranges, e.g., to change the ring laser gyroscope from one resonance to a fifth higher or lower resonance. For operation with visible red helium-neon laser wave-lengths, this means the mirror must be able to move at least ##EQU1##
Even though ultra-low expansion materials such as Schott Zerodur and Cervit 101 by Owens Illinois Corporation is used, the path length of a ring laser gyroscope will still experience a substantial change in the path length when experiencing a temperature change from -55.degree. C. to +70.degree. C. For example, if Schott Zerodur is used (expansion coefficient .alpha.=-8.times.10.sup.-8 /.degree.C.), the path length of a ring laser gyroscope having a typical roundtrip path length of 0.32 meters will experience a change in the path length by -3.2.times.10.sup.-6 meters for such a temperature change. That is, the path length will be decreased or shortened by that amount. Such a length change corresponds to 5 .lambda. (i.e. five wave length) with operation of the laser in the visible helium-neon transition. When the ring laser, using a prior art path length controller, is turned on at for example -55.degree. C., the initial voltage input to the actuator will probably be on the order of 0 volts. Furthermore, if as in the usual situation we assume the worst case condition, that the closed loop system locks on by increasing the total path length one-half free spectral range at 55.degree. C. it will be appreciated that the servo has used up this part of its total range. This will of course necessitate increasing the total range by one-half free spectral range. Therefore, the prior art necessary range of a piezoelectric actuator is ##EQU2## A comparison of equation (2) with equation (1) above, shows the increase of the one-half free spectral range. Modern piezoelectric materials are typically capable of changing their thickness according to the formula EQU .DELTA.L/L=200.times.10.sup.-6 ( 3)
at full applied voltage. Therefore, combining equation (2) and equation (3), it is seen that the required total thickness of a stack of piezoelectric discs is ##EQU3## Although as will be discussed later, there are specifically designed piezoelectric discs which may be considered "double acting" and therefore can reduce the length arrived at an equation (4) to one-half the normal value (or 0.005 meters) it will be appreciated that in any case, all of the prior art piezoelectric actuators essentially use only about one-half of the total available stroke when the initial voltage input to the actuator is 0 volts and the actuators closed loop system locks on as described above. This is because, as will be explained hereinafter, only one polarity of the piezoelectric actuator stack is used. Use of only one polarity and consequently one-half of the available stroke of the piezoelectric actuator stack presents a serious problem on how to compensate for such changes due to temperature. This means that either the temperature range must be limited or the thermal expansion coefficient must be kept even smaller than is now achieved by the ultra-low coefficient of expansion material now being used in ring laser gyroscopes. Alternately, the stroke of the piezoelectric actuator must be so large as to necessitate the use of an extremely thin flexible annular area or membrane in the mirror and perhaps the use of a bimorph piezoelectric actuator. It will be appreciated by those skilled in the art that a bimorph actuator acts similar to a bimetal system but is made of piezoelectric material with radially expanding discs. Typically, one expanding disc and one contracting disc is used. Unfortunately, such an arrangement has extremely low stiffness and if used in ring laser gyroscopes the membrane or thin annular ring in the mirror must be no greater than 0.4 millimeters. It will also be appreciated that such thinness greatly increases the price and the risk of mechanical failure. These choices obviously are just not satisfactory. For example, if a large stroke piezoelectric actuator is used (i.e. L=0.01 meters) it is quite possible the overall size of the ring laser gyroscope must be increased to the point it will be necessary to have a larger package around the unit. Furthermore, since piezoelectric discs are quite expensive, and since many more discs will be required if the length is increased, the overall price will be excessive. On the other hand, use of an extremely thin flexible mirror with a bimorph actuator is unacceptible since the annular area in the mirror is so expensive to manufacture because of necessary polishing, etc., and since it is sensitive to ambient pressure. In addition, such a combination arrangement is mechanically weak, subject to influence from vibrations, and is prone to unwanted simultaneous rotation as it provides the desired rectilinear motion. This simultaneous rotation will cause the laser beam inside the ring laser to shift its position with respect to the mirrors and apertures, and thus change the forward scattering in each beam. This causes the ring laser output to indicate a rotation. However, regardless of the type of piezoelectric actuator used, and regardless of the type of piezoelectric discs used, as was mentioned above, all of the present path length controllers use only about one-half of the total available stroke of the piezoelectric stack since only one polarity of the piezoelectric stack is used.
Therefore, to overcome the short comings of available methods, it is an object of this invention to provide methods and apparatus which use the full rectilinear motion available from a piezoelectric actuator to compensate for thermal expansion and contractions.
It is still another object of this invention to provide an inexpensive and simple piezoelectric actuator.
It is a further object of this invention to minimize the number of peizoelectric devices required in a piezoelectric actuator.
To accomplish the above mentioned objects as well as other objects which will become evident from the following drawings and detailed description, the present invention provides circuitry which allows substantially the entire stroke of a piezoelectric actuator to be used to accurately control a parameter which varies because of thermal expansion or contraction of a structure by presetting the stroke position of said actuator. A particular use is for accurately maintaining the path length of the laser beam of a ring laser gyroscope as a function of the startup temperature. Such circuitry comprises a temperature sensing means for determining the temperature of said structure and for providing an output representative of said temperature. The output from the temperature sensing means is received by a circuit network which provides a control signal which varies in response to the temperature sensing output and mathematical model of the expansion and contraction characteristics of the material from which the structure is made. The control signal is received by a driving means which provides the necessary driving voltage to position the piezoelectric actuator according to the value of said control signal. Once the actuator has been preset to the desired position then switching means switches the control circuitry from the presetting circuitry to the normal operating closed loop servo circuitry.